if '14 get one made July 23, 2013 or later. that's the "mid-year 2014.5 refresh" cut date which means the Adaptive Cruise Control has "w/stop" so the ACC will bring you to a complete stop and hold for 2-3 secs...very nice in slinky traffic...rigs built 7/22/13 and earlier release below 20mph I believe so mainly only for flowing traffic...anyway, this also keeps you away from some of the earlier builds that had more teething problems. 14 vs 15 Summit? eh, no critical diffs come to mind other than you should always get the youngest, healthiest vehicle in your price range. Also, in your neck of the woods you should double check you are not buying a Canadian built one as it will have zero warranty...first US registration chops the warranty to zip.
While you are searching you can get the on board equipment list and few options that exist on a Summit by plugging the VIN in here:
Good luck with the search...